first, change all the units to SI units, giving you:
3000 kg/m3 for the sphere's density
0.12 kg for its mass

to possibly calculate the sphere's weight underwater, you must calculate the net force of the sphere's weight downwards and buoyancy upwards

buoyant force is equal to the weight of the water displaced by the ball, thus you calculate the weight by using the mass of the water displaced, found by:
1. density of water
2. the volume of water displaced, which is equal to the volume of the sphere since it is fully submerged